Potential and Kinetic Energies
Potential Energy
Energy is the capacity to do work (scalar)
Potential energy (Ep) is the energy of an
object due to its position or condition
“Potential” means “stored”
ex. a battery has chemical energy,
food has chemical energy,
a compressed spring has elastic energy,
a raised object has gravitational energy
Dulku – Physics 20 – Unit 3 (Circular Motion, Work and Energy) – Topic I
Dulku – Physics 20 – Unit 3 (Circular Motion, Work and Energy) – Topic I
Dulku – Physics 20 – Unit 3 (Circular Motion, Work and Energy) – Topic I
Specific Outcomes:
i. I can determine, quantitatively, the relationships among the kinetic,
gravitational potential and total mechanical energies of a mass at any
point between maximum potential energy and maximum kinetic energy.
Potential Energy
Kinetic Energy
Conservative Forces
Potential and Kinetic Energies
Potential Energy
“Position” refers to a location or change in
location
ex. gravitational energy, elastic energy
“Condition” usually refers to a type of energy
ex. chemical energy
We will focus on gravitational potential
energy here
Dulku – Physics 20 – Unit 3 (Circular Motion, Work and Energy) – Topic I
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Potential Energy
The gravitational potential energy equation:
where:
Ep = gravitational potential energy (J)
Ep = mgh
Potential Energy
ex. Calculate the potential energy possessed by a 20 kg
mass at a height of 1.00 m.
Ep = mgh = (20 kg)(9.81 m/s2)(1.00 m)
= 2.0 x 102 J
m = mass of the object (kg)
g = magnitude of acceleration due to
gravity (m/s2) (9.81 m/s2 on Earth)
h = height of the object (m)
Dulku – Physics 20 – Unit 3 (Circular Motion, Work and Energy) – Topic I
Dulku – Physics 20 – Unit 3 (Circular Motion, Work and Energy) – Topic I
Potential Energy
ex. Determine the height of a 154 g ball, if it has a
potential energy of 50 J.
m = 154 g ÷1000 = 0.154 kg
h=
Ep = mgh
Ep
50 J
=
= 33 m
mg
(0.154 kg)(9.81 m/s2)
Dulku – Physics 20 – Unit 3 (Circular Motion, Work and Energy) – Topic I
Kinetic Energy
Kinetic energy (Ek) refers to the energy of an
object’s motion, related to speed or velocity:
Ek = ½mv2
OR
mv2
Ek =
2
where:
Ek = kinetic energy (J)
m = mass of the object (kg)
v = speed of the object (m/s)
Dulku – Physics 20 – Unit 3 (Circular Motion, Work and Energy) – Topic I
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Kinetic Energy
ex. Calculate the kinetic energy of a 1200 kg car
traveling at 20 m/s.
Ek = ½mv2 = ½(1200 kg)(20 m/s)2
= 2.4 x
105
J
ex. How quickly is a 400 kg car with 5000 J of kinetic
energy moving?
v=
Conservative Forces
A force may transform (change) one form of
energy into another form
Potential energy may be transformed into
kinetic energy, or vice versa
There are two classes of force: conservative
and non-conservative
2Ek
2(5000 J)
=
= 5.00 m/s
m
400 kg
Dulku – Physics 20 – Unit 3 (Circular Motion, Work and Energy) – Topic I
Conservative Forces
A conservative force is independent of the
path used to transform the energy
Dulku – Physics 20 – Unit 3 (Circular Motion, Work and Energy) – Topic I
Conservative Forces
If an object drops from a height, its potential
energy is converted into kinetic energy
ex. gravitational force depends on height,
not how that height is changed
At maximum height (at rest), an object has
only potential energy
A non-conservative force depends on the
path or direction taken
At impact with the ground (highest speed),
an object has only kinetic energy
ex. friction force is always opposite to the
path of motion
Dulku – Physics 20 – Unit 3 (Circular Motion, Work and Energy) – Topic I
From maximum height to impact, potential is
completely transformed into kinetic energy!
Dulku – Physics 20 – Unit 3 (Circular Motion, Work and Energy) – Topic I
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